Foundation for a wind turbine

ABSTRACT

This invention comprises a wind turbine foundation with a hollow central body in the shape of a truncated cone which has an upper flange or a pedestal to connect with the tower (not shown), and a lower slab in the shape of a plane ring. In the preferred embodiment, the foundation leans the interior surface of the cone and the lower slab on earth from the local terrain, which takes on the role of permanent formwork; in an alternative embodiment, the interior of the foundation is left hollow and accommodates components of the wind turbine within it; and in a final alternative embodiment, the foundation is placed over the surface of the terrain which supports only the base of the lower slab, the central body not being buried.

FIELD OF THE INVENTION

This invention relates in general terms to a wind turbine foundation in the shape of a cone, and more particularly a foundation which comprises a central body in the shape of a truncated cone over which is found an upper flange or a pedestal for connecting with the tower and a lower slab in the shape of a flat ring.

BACKGROUND

The power generated in a wind turbine is proportional to the size of the rotor. Therefore, to generate more power, the size of the blades is increased, and as a consequence there is an increase in the size of the power train, of the turbine and the height of the tower. It follows that there must also be an increase in dimensions of the foundation upon which the wind turbine is supported, up to the point where a reduction in the material used in the foundation leads to a considerable reduction in the cost of the wind turbine unit. To this end the above technique offers a number of foundation options to meet ever more challenging requirements and in turn reduce the material used and/or facilitate installation:

Publication WO2004101898A2 shows two alternatives for a circular foundation with prefabricated triangular sections which is buried, using the earth from the excavation, once installed. The first design involves 12 prefabricated pieces which are joined together to form the foundation, and the second variant involves the union of 10 circular segments, spaced out from each other to save on material, and a central body.

Publication DK200100030 considers a star-shaped foundation with 12 prefabricated triangular sections which are supported on a base and buried using the earth from the excavation.

Publication NL1024581 defines a circular variable depth foundation with radial and tangential reinforcements which are symmetrical with respect to the central axis.

Publication EP1074663 is an example of a star-shaped foundation with three stabilisers placed symmetrically around a central support, made up of a set of rods in its peripheral sections.

Publication U.S. Pat. No. 6,672,023 concerns a foundation which comprises a cylindrical wall over which rests a circular base and a pedestal with means of attachment to the tower.

The wind power industry is ever demanding foundations optimised for the loads required to support wind turbines, and this invention is intended to meet that demand.

SUMMARY OF THE INVENTION

Currently the most commonly used designs for wind turbine foundations are foundation slabs of various sections (square, circular, hexagonal, octagonal). More particularly, in the majority of cases for wind turbines with a power rating below 1 MW square section foundation slabs have been used since this is a very well known solution, easy to design and calculate, and further its simplicity makes formwork and construction easier. However as the power of wind turbines has increased, the following disadvantages of this type of square slab foundation have started to manifest themselves and take on more importance:

-   -   The weight is contributed by the mass of the concrete, in other         words, by one of the most costly parts of the foundation.     -   Inertia does not play an important part, since we are concerned         with a continuous section.     -   A large volume of earth fill is generated which has to be         evacuated, giving rise to an environmental and economic problem.     -   It is a solution with low structural efficiency, which is not         well suited to the distribution of stresses. Stresses in the         sections of the foundation reduce with distance from the centre         of the foundation, therefore a smaller section is needed at the         extremities of the foundation.

As the power of wind turbines increases, typically we see a scaling up of solutions along with which some alternatives appear which are not optimal for multimegawatt wind turbines of the type considered state of the art. Specifically, for wind turbines with power ratings greater than 2 MW, the foundation designs commonly in use start to become uncompetitive in comparison with other more structurally appropriate designs.

The table below sets out estimates of the foreseeable progression of the total cost of a square section slab foundation with increasing wind turbine power, as well as an estimate of the cost/kW.

Power Total Cost Cost (MW) (

) (

/KW) 0.85 25,189 29.6 2 60,173 30.1 4 167,783 41.9 6 259,116 43.2 8 441,960 55.2 10 874,952 87.5

As it can be seen from the table, the cost of a solid square footing increases exponentially as higher powered wind turbines are installed. At the same time the cost/KW of the above-mentioned footing also increases exponentially, although not as sharply, with increases in wind turbine power. The trends in this table are easy to extrapolate to other well-known solutions, which shows that other designs can be more competitive than the current ones in relation to multimegawatt wind turbines.

From the structural point of view, analysing the typology of the loads at the base of the tower which the wind turbine foundations have to support, they can be summarised as a bending moment, which is critical to the calculation, and an axial force in the direction of the longitudinal axis of the tower. To counteract these loads there are two alternatives which can be pursued:

-   -   Foundations based on gravity, where it is necessary to use         weight, looking to achieve volume (gravity foundations such as         foundation slabs of various cross sections).     -   Foundations based on inertia, where it is necessary to include         components which provide inertia by involving radii or sides         (star-shaped foundations, etc.).

After analysing the various possible foundation designs for large wind turbines it has been possible to conclude that the application of inertia foundation designs could be ideal for high-power wind turbines (>4 MW) in comparison with scaling up the gravity foundation solutions in current use.

The aim of this invention is to provide a wind turbine foundation with a central body in the shape of a truncated cone which is formed in concrete over the earth of the terrain, and which has a lower slab in the shape of a plane ring.

Another aim of an alternative embodiment of this invention is to have a wind turbine foundation with a central body in the shape of a truncated cone whose interior houses the electrical installations of the wind turbine or other components of the latter, and which has a lower slab in the shape of a plane ring.

And another aim of an alternative embodiment of this invention is a wind turbine foundation with a central body in the shape of a truncated cone which has a lower slab in the shape of a plane ring which sits directly on the surface of a terrain with great load-bearing capacity, without needing to bury the central body, in whose interior there can also be housed a series of wind turbine components.

One of the advantages of the preferred embodiment of this invention is the optimisation of the total foundation cost for wind turbines rated at 2 MW and above in comparison with the state of the art, due principally to the saving in materials (concrete, reinforcing steel) relative to conventional foundation designs; the reduction in the volume of excavation, which reduces the cost of excavation in addition to achieving a significant reduction in cost of transport to the waste dump (economical and environmentally friendly solution); and the reduction and simplification of the formwork since the local earth itself is used, rather than evacuated from the central body, as permanent formwork during the forming in situ of the foundation concrete.

It must also be highlighted that with the preferred embodiment there is an increase in the safety margin and the rigidity of the foundation due to the action of the terrain which has not been excavated and which remains housed in the hollow of the cone, absorbing part of the load on the wall of the cone, and because the earth which is used to bury the cone from the outside provides increased stability to the structure of the foundation.

The following table sets out an estimate of the progression of the total cost and cost/kW of the hollow cone foundation for various power ranges. It illustrates a clear reduction in the cost of foundation with the wind turbines of the order of 2-10 MW compared with the conventional alternatives mentioned above:

Power Total Cost Cost (MW) (

) (

/KW) 0.85 32,693 38.5 2 60,431 30.2 4 131,342 32.8 6 171,137 28.5 8 268,544 33.6 10 457,214 45.7

Another advantage of this invention is the increase in stability due to the inertia provided by the diameter of the base of the cone, compared with the corresponding stability provided by state of the art foundations having the same mass. Further it improves the capacity to absorb wind turbine loads since the flow of stresses is dispersed throughout the length of the walls of the cone as they travel downwards through the foundation in a smooth and uniform manner, independently of the orientation of the nacelle and the directions of the above-mentioned loads: As the nacelle changes its orientation to match the direction of the wind in order to optimise the production of energy, the direction of the forces changes with the rotation of the nacelle.

Another advantage of this invention is the suitability of the foundation for different ranges of wind turbine loads and the different kinds of terrain upon which they can be installed since the following design elements are easy to modify:

-   -   The slope of the conical surface can be changed to transmit         stresses to the terrain in a different manner. On occasions it         may be necessary to increase the diameter of the base of the         cone to reduce the stresses occurring in the wall of the conical         surface.     -   The upper part of the conical surface can be used to house         different types of interface with the tower (embedded section of         foundation, post-tensioned bolts, etc.), which on its lower part         could be tubular steel or concrete.

Another advantage of this invention is the greater structural efficiency at 80000 KNm and above at the base of the tower. That is a better relationship between the diameter, the material and the geometry or the structural configuration, in comparison with the state of the art as currently published.

Another advantage of an alternative embodiment, is the saving in the excavation itself for those terrains whose capacity for support and properties are such that it is not necessary to bury the conical foundation considered in this invention so that it can be used as a base for the tower without the need for additional foundation. Both this solution and the alternative embodiment, where the foundation is buried evacuating the earth from the interior of the cone, enable the foundation to be implemented in a modular fashion with prefabricated pieces which facilitate transport and assembly. Once installed, the above-mentioned space can be used to house the wind turbine's electrical installations or other components thereby optimising the interior space in the tower and nacelle.

Other features and advantages of this invention can be seen from the following detailed description of an illustrative implementation which is not limitative in so far as the accompanying FIGURES are concerned

BRIEF DESCRIPTION OF FIGURES

FIG. 1 shows a cross section of the foundation as per this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

With this invention the conical foundation is based on a conical shape with a lower slab (2) which receives the stresses from the walls of the cone (1) and transmits them to the ground. Sometimes a pedestal which is not shown can be used to connect the foundation to the tower, at the upper part of the cone, or a concentric ring in the form of a flange to allow access to the hollow interior of the foundation in the case of other embodiments.

In a similar way to inertia foundations, in this case the inertia is supplied by the radius of the cone at its base, and the transfer of stresses from the lower part of the tower to the supporting ground can be achieved smoothly, on being transmitted by the conical surface.

There are 2 main variants and one secondary variant of this design:

-   -   The filled conical foundation considered in the preferred         embodiment of this invention. As the earth is not extracted from         the interior, the walls of the conical surface (1) are supported         on the above-mentioned earth, so that smaller stresses are         produced in the walls, since local bending is avoided.         Therefore, the thickness of these walls is optimised and further         an additional saving is achieved in the transport and movement         of the surplus fill earth.     -   And the hollow conical foundation, where the fill earth is         extracted from the interior of the cone (1). The walls of the         conical surface take greater stresses due to local bending, so         the thickness is increased. The empty space resulting from the         extraction of earth can be used to house electrical enclosures,         transformers and other types of devices or installations (eg:         lifting machinery, etc.).     -   Additionally we also consider an embodiment of a wind turbine         base with the same design for use in terrains with a great         supporting capacity, in which the lower slab (2) supports its         base directly on the ground with no need for any excavation.         Similarly to the previous case, the walls of the conical surface         are thicker and the radius of the lower slab (2) is also greater         than under the previous alternatives.

The main new feature and advance under the preferred embodiment of this invention lies within the design of a conical foundation which takes advantage moreover of the earth which is not extracted for use as permanent formwork while the concrete is poured in situ. This fact allows the design of the foundation to be optimised, reducing the quantity of material used and lowering the cost of foundations, in addition to the advantages referred to in the preceding section.

In any instance where the preferred embodiment is not ruled out, the foundation is made up of a plane ring (2) for the base and a truncated cone shaped body in the central part (1) which is formed in concrete on the unexcavated terrain.

The cross section of the plane ring (2) is preferably rectangular, but additionally the upper and/or lower side could have a slight slope between 0 and 45° relative to the horizontal. The dimensions of the above-mentioned ring can have, depending on the load and dimensions of the wind turbine, a radius (R) of up to 18 m, a thickness (L) of up to 6 m, and a height (c) of up to 1 m. In a preferred embodiment but not ruled out under this invention, and other than as shown in FIG. 1, the lower ring (2) is joined to the central conical body (1) in the interior area of the ring (2) so as not to leave a projection into the interior, with the aim of facilitating excavation work, formwork and the pouring of concrete.

The section of the conical body (1) of the preferred embodiment, is produced via a generatrix in the form of a rhomboid with the two short sides parallel to the horizontal at its upper and lower extremes and the two long sides parallel at an inclination falling preferably between 20 and 60° with respect to the horizontal. The thickness (e) defined by the distance between the above-mentioned long sides of the generatrix is preferably constant and depending on requirements reaches values up to 1.4 m. Alternatively, the thickness of the cone can increase (1) as we approach the lower ring (2). In an alternative to the preferred embodiment, additionally the above-mentioned long sides of the generatrix are formed from complex curves with a large radius of curvature, which produces a foundation in the shape of a cone with pseudo-spherical walls.

The total height of the foundation (H) reaches values up to 6 m, and the diameter (Dp) of the upper mouth of the central body reaches dimensions up to 15 m in diameter depending on the diameter of the tower and the load from the wind turbine.

The dimensions of the foundation will vary depending on the load bearing capacity of the earth and also on the load from the wind turbine. However, by way of example, for a wind turbine of the order of 2 to 4 MW the ranges of the principal dimensions are:

-   -   H: between 2.5 and 4.5 m preferably     -   R: between 8 and 14 m preferably     -   Dp: between 4 and 12 m preferably     -   L: between 2.5 and 5 m preferably     -   c: between 0.25 and 0.6 preferably     -   e: between 0.25 and 1.2 m preferably

Initially the conical foundation design is intended to be executed in situ with conventional concrete having characteristic strength between 25 and 35 MPa.

Alternatively, this design can be carried out using high-strength concrete, special concrete, fibre reinforced concrete. Another alternative is provided by prefabricated concrete components with different qualities of concrete for the different components (conical surface, lower slab, etc.).

Within the process of pouring concrete in situ, in the case of the preferred embodiment with the hollow filled with earth, the excavation must be carried out taking account of the final volume of the conical foundation. Once the excavation is carried out, a layer of mud slab is placed on the ground, and upon that the previously pre-mounted reinforcements, for subsequent pouring of concrete. Depending on the slope of the conical surface, it may be necessary to use formwork.

In the case where prefabricated components are used we open up a wider range of shapes for the foundation, since the rhomboid which characterises the generatrix of the hollow cone (1) can easily change its long sides to produce pseudo-spherical surfaces in the walls of the cone (1), or alternatively to combine straight lines with curves in the two long sides of the generatrix. In this case, it is also necessary to use cables to pre-tension or post-tension the above-mentioned prefabricated components in the axial and/or radial directions of the foundation.

In the alternative embodiment of the hollow variant, the concrete pouring for the conical surface (1) requires the use of interior formwork, which must be suitably underpinned and held together, either that or make use of prefabricated components for ease of transport and assembly as described above.

The calculations for the dimensions and costs of the foundation as set out above are based on the study of modelling by finite elements, the quantity of reinforcements and tensile strength required for the case of the preferred embodiment where concrete is poured in situ being deduced from a study of the distribution of stresses in the most unfavorable section nodes in each model of the wind turbine. In the comparative study between the various designs, there has also been an analysis of the main components of cost (blinding concrete, structural concrete, reinforcements, formwork, earth fill and compaction, excavation and transport to the waste dump, etc.) to produce the tables shown above. 

1. Wind turbine foundation with means of attachment to the tower at its upper part characterized because it consists of a solid of revolution in the shape of a hollow cone (1) whose interior surface is seated upon permanent formwork of non-excavated earth in the shape of a cone, and which has a second solid of revolution in the shape of a ring (2) at its lower base.
 2. Wind turbine foundation in accordance with claim 1 characterized because the generatrix of the cone (1) is a surface in the shape of a rhomboid, with two short sides parallel to the horizontal and the other two longer sides formed preferably by straight lines which are angled relative to horizontal between 20 and 60° degrees, and are separated by a distance (e) of 0.25-1.2 m between them.
 3. Wind turbine foundation in accordance with claim 1 characterized because the lower ring (2) has a radius (R) between 8 and 14 m, a width (L) between 2.5 and 5 m and a height (c) between 0.25 and 0.6 m.
 4. Wind turbine foundation in accordance with claim 1 characterized because the upper mouth of the cone has a diameter (Dp) between 4 and 12 m.
 5. Wind turbine foundation in accordance with claim 1 characterized because the total height (H) is between 2.5 and 4.5 m.
 6. Wind turbine foundation in accordance with claim 1 characterized because either the two long sides of the generatrix are curves, or alternatively they are formed by the combination of one long straight side and one long curved side.
 7. Wind turbine foundation in accordance with claim 1 characterized because alternatively the thickness (e) of the wall of the cone (1) varies with depth in the direction of the base (2).
 8. Wind turbine foundation in accordance with claim 1 characterized because either the upper and/or lower surface of the ring has an inclination relative to the horizontal between 0 and 45°, and/or because alternatively the central body (1) connects with the interior radius of the ring (2) so that there is no projection into the interior of the cone (1).
 9. Wind turbine foundation in accordance with claim 1 characterized because alternatively the earth in the hollow interior of the cone (2) is evacuated and used to house components of the wind turbine in the resulting free space.
 10. Wind turbine foundation in accordance with claim 1 characterized because alternatively the lower plane ring (3) lays down directly on the surface of the ground. 